SEAMARC II EVIDENCE FOR THE LOCUS OF SEA-FLOOR SPREADING IN THE SOUTHERN MARIANA TROUGH

Two surveys near 13-degrees-45'N across the Mariana Trough identify the axis of backarc seafloor spreading at 143-degrees-53'E. The spreading center is identified on the basis of its high acoustic backscatter in SeaMARC II side-scan images and by the presence of young volcanic features. Neither asymmetric spreading nor discrete jumps in the spreading center location are required to explain the observed morphology and backscatter characteristics. Changes in the orientation of the seafloor spreading fabric suggest that an early phase of arc rifting may have been followed by northward propagation of the backarc spreading center to widen this portion of the basin. This could have produced the current situation, with the rift axis offset about 35 km east of the center of the Mariana Trough. The off-center position of the spreading axis may also be the result of overgrowth of backarc crust by the volcanic arc.